Conventionally electrodeposited copper foil is used in various fields such as rigid printed circuit boards, flexible printed circuit boards, and electromagnetic field shielding materials.
Within these fields, in the fields of flexible printed wiring board (hereafter referred to as “FPC”) laminated with polyimide film, there is a demand for higher strength copper foil for hard disk drive (hereafter referred to as HDD) suspension materials, and for tape automated bonding (hereafter referred to as “TAB”) materials.
In the case of suspensions in HDDs, with the progressive increase in capacity the wire type suspensions that were conventionally used have been mostly replaced by the wiring integrated type suspension which has high stability of suspension force and positional accuracy for the disk, which is the recording medium.
This wiring integrated type suspension includes three types: (1) the type referred to as the flex suspension assembly (FSA) method in which a flexible printed board is processed and bonded using adhesive, (2) the type referred to as the circuit integrated suspension (CIS) method in which amic acid, a precursor of polyimide resin, is processed to a shape, then it is converted to polyimide and a plating process is performed on the polyimide obtained to form the wiring, and (3) the type referred to as the trace suspension assembly (TSA) method in which a 3 layer laminate structure of stainless steel foil, polyimide resin, and copper foil is formed into predetermined shape by etching.
Of these, the TSA method suspension can easily form flying leads by laminating copper foil onto the stainless steel foil which has high strength, and because it has a high degree of freedom of shape processing, is comparatively low cost, and has good dimensional accuracy, it is widely used.
In suspensions formed by the TSA method the laminate is manufactured using as materials stainless steel foil with a thickness of about 12 to 30 μm, a polyimide layer with a thickness of about 5 to 20 μm, and copper foil with a thickness of about 7 to 14 μm.
In the manufacture of the laminate, a liquid that includes a polyimide resin precursor is applied to the stainless steel foil, which is the substrate. After application, solvent is removed by preliminary heating, and then further heat processing is carried out to form polyimide, then the copper foil is superimposed on the polyimide resin layer that has been converted to polyimide and laminated by thermal bonding at a temperature of about 300° C., to produce the laminate made from the stainless steel layer/polyimide layer/copper layer.
In this heating to 300° C. there is virtually no dimensional change in the stainless steel foil. However, when conventional electrodeposited copper foil is used, the electrodeposited copper foil is annealed at 300° C., and recrystallization causes progressive softening, thereby producing dimensional changes. Therefore after laminating, there is warping of the laminate, which is a problem for the dimensional accuracy of the product.
In order that warping is not produced in the laminate after laminating, it is necessary that the dimensional changes of the copper foil during heating be made as small as possible, and 0.1% or less is required.
Conventionally rolled copper alloy foil is used as the copper foil to satisfy this requirement. Rolled copper alloy foil is not annealed at a temperature of 300° C., its dimensional change during heating is small, and the change in mechanical strength is also small.
Rolled copper alloy foil is produced by rolling copper alloy that includes copper as the main component, and also includes one or more element apart from copper such as tin, zinc, iron, nickel, chromium, phosphorous, zirconium, magnesium, silicon, or the like, to produce the foil. Depending on the type of element and their combination, these rolled copper alloy foils are not annealed at about 300° C., and there is not much change in their tensile strength, 0.2% proof stress, elongation, etc.
For example, rolled copper alloy foil such as Cu-0.2 mass % Cr-0.1 mass % Zr-0.2 mass % Zn (Cu-2000 ppm Cr-1000 ppm Zr-2000 ppm Zn) is ideally suited for use as a HDD suspension member, as wells as TSA method suspensions.
High strength copper foil is also required for TAB materials, the same as for TSA method suspensions and HDD suspension materials.
In TAB products, a plurality of terminals of an IC chip are directly bonded to inner leads (flying leads) disposed in a device hole located in substantially the center of the product.
At this time the bonding is carried out using a bonding device (bonder), which instantaneously heats by passing an electric current, and applies a fixed bonding pressure. At this time, there is the problem that the inner lead which is obtained by etching the electrodeposited copper foil is pulled and extended too much by the bonding pressure.
By increasing the strength of the electrodeposited copper foil, slackness and rupture of the inner lead is inhibited. Therefore if the strength of the electrodeposited copper foil is too low, slackness of the inner lead is produced by plastic deformation, and in the extreme case there is the problem that the foil is ruptured.
On the other hand, by reducing the roughness of the copper foil surface, when forming the TAB wiring by etching, it is possible to prevent excessive thinning due to over-etching the side walls of the wiring, so it is possible to form narrow wiring by etching. The reason for this is as follows: normally HDD and TAB wiring is formed by thermal lamination of copper foil to a polyimide substrate, and then a resist is applied and etching is carried out. At this time, if the surface roughness of the copper foil is coarse, the copper foil cuts into the polyimide surface, and when the portion that was cut into is removed by etching, the side walls of the wiring are over-etched and the wire width is narrowed. In order to avoid this it is desirable that the copper foil surface should have a low roughness.
Therefore, in order to achieve narrow width inner leads, it is necessary that the electrodeposited copper foil used have low roughness and high strength.
In this case as well, it is necessary that the copper foil or copper alloy foil have high strength under normal conditions (25° C., 1 atmosphere, and hereafter also), and have high strength after heating.
For TAB applications, a two layer FPC in which the copper foil is laminated to the polyimide layer or a three layer FPC in which the copper foil is bonded to the polyimide layer with an adhesive layer are used. In the three layer FPC when bonding the copper foil to the polyimide an epoxy adhesive is used, and the bonding is carried out at a temperature of around 180° C. In the two layer FPC using a polyimide adhesive, bonding is carried out at around 300° C.
Even assuming the copper foil has high mechanical strength under normal conditions, this strength is of no benefit if it softens when bonding to the polyimide. The conventional high strength electrodeposited copper foil has high mechanical strength under normal conditions, and even when heated to about 180° C. there is almost no change in mechanical strength, however, when heated to about 300° C. annealing occurs, softening occurs rapidly as recrystallization progresses, and the mechanical strength is significantly reduced.
The following is a description of various researches that have been carried out into electrodeposited copper foil with low roughness on the surface of the copper foil that is bonded to the polyimide resin substrate (normally the rough surface of the copper foil), and with excellent mechanical strength.
For example, Patent Document 1 describes a low surface roughness electrodeposited copper foil with a uniformly low surface roughness Rz of 2.0 μm or less and with an elongation of 10.0% or more at 180° C. that is ideal as copper foil for printed wiring board applications and lithium secondary battery negative electrode current collector applications.
The above electrodeposited copper foil can be obtained with an aqueous solution of sulfuric acid-copper sulfate as the electrolyte, in the presence of polyethyleneimine or its derivative, a sulfonate salt as active organic sulfur compound, chlorine ion (chloride ion) at a concentration of 20 to 120 mg/L, and an oxyethylene surfactant with a prescribed concentration.
Also, Patent Document 2 describes an electrodeposited copper foil having a surface roughness Rz of 2.5 μm or less, whose tensile strength measured at 25° C. within 20 minutes of completion of electrodeposition is 820 MPa or higher, and whose reduction in tensile strength at 25° C. measured at 300 minutes after completion of electrodeposition is not more than 10% of the tensile strength at 25° C. measured within 20 minutes of completion of electrodeposition.
The above electrodeposited copper foil can be obtained with an aqueous solution of sulfuric acid-copper sulfate as the electrolyte, in the presence of hydroxyethyl cellulose, polyethyleneimine, a sulfonate salt as an active organic sulfur compound, acetylene glycol, and chloride ion at a concentration of 20 to 120 mg/L.
In addition, Patent Document 3 describes a controlled low profile electrodeposited copper foil having substantially no columnar grains and twin boundaries, and having a grain structure with mean grain size up to 10 μm, in which the grain structure is substantially uniform with a random grain orientation.
This electrodeposited copper foil has a maximum tensile strength at 25° C. in the range of 87,000 to 120,000 psi (600 MPa to 827 MPa), and at 180° C. has a maximum tensile strength in the range of 25,000 to 35,000 psi (172 MPa to 241 MPa).
Patent Document 4 describes a method for manufacturing electrodeposited copper foil using a sulfuric acid acid copper sulfate electrolyte to which tungsten or a tungsten compound, glue, and chloride ions at a concentration of 20 to 120 mg/L are added. As a result the hot elongation at 180° C. is 3% or more, the roughness of the rough surface is large, and it is possible to manufacture copper foil with few pinholes.
Therefore the present inventors and others carried out repeated tests on electrodeposition using an electrolyte in which tungsten or tungsten compound is added to a sulfuric acid-copper sulfate electrolyte, and glue and chloride ion to a concentration of 20 to 120 mg/L are also added, and Patent Document 4 confirmed that it is possible to manufacture copper foil in which the hot elongation at the target 180° C. is 3% or more, the roughness of the rough surface is large, and there are few pinholes. However, as a result of analyzing this copper foil, it was determined that there was no eutectoid reaction with tungsten in this electrodeposited copper foil. In other words, it was not possible to obtain electrodeposited copper alloy foil (copper-tungsten copper alloy foil) (see Comparative Example 4 which is described later).
Therefore, in the method described in Patent Document 4, it is not possible to manufacture electrodeposited copper alloy foil having low roughness on the rough surface, and high mechanical strength under normal conditions, with little reduction in mechanical strength at high temperatures when heated.
An explanation of the reasons for this is described later.
Also, Patent Document 6 describes a dispersion strengthened electrodeposited copper foil where the copper exists as fine crystal grains in which very fine grains of SnO2 are dispersed.
In Patent Document 6, copper ions, sulfate ions, tin ions, and organic additives such as polyethylene glycol, and so on, are added to a sulfuric acid copper sulfate electrolyte, ultra fine particles of SnO2 are generated by bubbling a gas containing oxygen into the electrolyte, and a dispersion strengthened electrodeposited copper foil is obtained using this electrolyte.
In addition, Patent Document 7 describes an electrodeposited copper foil that includes silver (Ag).
Patent Document 7 describes obtaining an electrodeposited copper foil using sulfuric acid copper sulfate electrolyte to which a silver salt having a predetermined concentration of silver ions was added. In this electrodeposited copper foil silver exists as a eutectic.